Scroll compressors have a wide range of applications where low to moderate compression ratios are desired, especially in the air conditioning and heat pump industries. This acceptance is attributed to high efficiency, fewer parts, and less noise and vibration when compared with competing compressors. A conventional scroll compressor includes a motor, which drives a shaft with an eccentric crank, causing orbiting motion of an orbiting scroll element. The orbiting scroll element has a scroll or spiral shaped protruding wrap, which interacts with a similarly shaped protruding wrap on a mating fixed element. Compression is achieved when the meshing coaction between the two protruding wraps shifts the gaseous fluid radially inward and simultaneously reduces the volume of the fluid.
However, internal leakage of pressurized fluid reduces the efficiency of scroll compressors. There are two types of leakage associated with scroll compressors, one is flank leakage, and the other is tip leakage. In both cases, the fluid in higher pressure pockets escapes through the gaps into lower pressure pockets. Flank leakage occurs when fluid from a pocket formed between the two protruding meshing wraps escapes at the flank surfaces where they come into contact with each other. Tip leakage occurs when fluid escapes between the end surface of the protruding wrap of each element and the base of the other element as they come into contact. Tip leakage is the more severe of the two because the effective total leakage path width for tip leakage is typically several times larger than that for flank leakage. Further, the compression process produces large axial loads that push the orbiting scroll element axially away from the fixed scroll element, thereby increasing the tip leakage. In addition to the axial forces driving orbiting scroll element away from the fixed scroll, there is also an overturning moment attempting to tip the orbiting scroll element out of the plane with the fixed scroll element.
This overturning moment results from the couple established between the non-axial component of the pressure forces generated within the compression pockets during the compression process and the reaction force thereof established between the shaft of the orbiting scroll element and its support bearings.
Since close-tolerance manufacturing techniques are not adequate to prevent the loss of pressure due to tip leakage, other methods have been developed. One approach is to utilize various types of tip seals, as described in U.S. Pat. Nos. 4,395,205; 4,411,605; 4,415,317; 4,416,597. The end surface of the protruding wrap of either scroll element is equipped with tip sealing means which reduce the tip leakage. Although this method is effective for sealing, it requires complicated manufacturing, increases friction, and raises costs.
Another approach to decrease tip leakage is to apply compensating back pressure to force mating elements together. Higher pressure fluid is purposely bled from the compression chamber through a vent port into a back chamber, which is typically a single, relatively large chamber located behind the orbiting scroll. This provides a body of pressurized fluid which pushes the orbiting element against the fixed element and thus, reduces the gap between the tips of the protruding scrolls and the bases of the elements. Reducing the gap minimizes the leakage of fluid, resulting in the increase of pressure in the compression chamber.
For example, U.S. Pat. Nos. 4,384,831; 4,600,369; 4,645,437; 4,696,630; and 4,861,245, each disclose a scroll compressor having such a back chamber. Commonly-assigned U.S. Pat. Nos. 4,992,032 and 4,993,928 also disclose scroll compressors using the back pressuring technique. As disclosed therein, rather than a single back chamber, two sealed pressure chambers, one at intermediate pressure and another at discharge pressure, are disposed behind the orbiting scroll element and are designed to counteract the gas compression forces within the compression chamber and to bias the orbiting scroll element toward the fixed scroll element. However, the prior art back pressuring technique is designed to overcome the highest overturning moment experienced during the orbiting cycle and leads to excessive thrust force over the remainder of the cycle. The large thrust force causes excessive friction between the two mating parts and results in reduced efficiency of the scroll compressors.
Additionally, U.S. Pat. No. 4,557,675 discloses a method of adjusting pressure in the back chamber by positioning pressure-equalizing ports so that the pressure vented into the back chamber varies with changes in operating conditions. However, the back pressure remains relatively constant during any given steady-state condition, thus, the change in pressure, as the operating conditions vary, is intended to overcome the highest overturning moment and axial force, resulting in excessive thrust force during the remainder of the cycle and causing excessive friction, thereby reducing the efficiency of the scroll compressor.